[feverittm]

There are also a couple of other possibilities (at least):

1 - The wheel shaft could be slipping in the encoder causing an changing number of ticks per revolution.

2 - The encoder wheel could be damaged (scratched). This can cause ticks to be missed or miscounted. Not at all deterministic.

We had these issues last year with our arm and needed to switch to a pot to measure arm angle. Not that a pot is that much more reliable.